Large anisotropic thermal conductivity of intrinsically two-dimensional metallic oxide PdCoO$_2$

TL;DR

This study reveals that the metallic oxide PdCoO₂ exhibits highly anisotropic electrical conductivity but relatively isotropic thermal conductivity due to significant phonon heat transport, with implications for understanding heat and charge transport in 2D materials.

Contribution

It provides detailed measurements of anisotropic thermal and electrical transport in PdCoO₂, highlighting the role of phonons in thermal conductivity despite electronic anisotropy.

Findings

Thermal conductivity is less anisotropic than electrical resistivity.

Phonon heat transport significantly contributes to thermal conductivity.

In-plane thermoelectric power shows a linear temperature dependence with a phonon drag peak.

Abstract

The highly conductive layered metallic oxide \pdcoo{} is a near-perfect analogue to an alkali metal in two dimensions. It is distinguished from other two-dimensional electron systems where the Fermi surface does not reach the Brillouin zone boundary by a high planar electron density exceeding $10^{15}$ cm$^{-2}$. The simple single-band quasi-2D electronic structure results in strongly anisotropic transport properties and limits the effectiveness of electron-phonon scattering. Measurements on single crystals in the temperature range from 10-300K show that the thermal conductivity is much more weakly anisotropic than the electrical resistivity, as a result of significant phonon heat transport. The in-plane thermoelectric power is linear in temperature at 300\,K and displays a purity-dependent peak around 50K. Given the extreme simplicity of the band-structure, it is possible to identify this peak with phonon drag driven by normal electron-phonon scattering processes.